1. Field of the Invention
The present invention relates to a charged particle beam apparatus, and more particularly to a charged particle beam apparatus that displays a scanning image of a substrate produced using a charged particle beam, fluctuations in the charged particle beam irradiation position, fluctuations in a stage position, or their respective power spectra, and evaluates the stability of the charged particle beam irradiation position, as well as to a displacement detecting circuit.
2. Background Art
Recently, with improvements in the levels of integration of semiconductor devices, there is a demand on CD measurement SEMs to have the ability to measure pattern shapes with accuracies of one nanometer or less. Further, there is a demand on defect review SEMs to have a function that detects minute defects of several tens of nanometers.
Regarding both measuring apparatuses and inspecting apparatuses, in many cases the image quality of an acquired image is determined based on contrast and edge sharpness and the like. However, in such cases it is difficult to quantitatively evaluate the stability of an electron beam position. Measurement of an electron beam irradiation position is itself difficult, and it is also difficult to measure positional fluctuations of a stage on which a substrate is mounted at a resolution of one nanometer or less without using a laser length measuring board that has a high resolution.
One known example of measuring an electron beam position is a method of measuring the electron beam position with a knife edge or a pattern edge (for example, see Japanese Patent Laid-Open Publication No. 11-016815 A1). According to the knife edge method, electron beam positions crossing an edge are evaluated by measuring electrons that are not obstructed by the edge, with a Faraday cup below the substrate. In contrast, a method in which a pattern edge is irradiated with an electron beam detects and evaluates secondary electrons that change based on the relation between the pattern edge and the beam diameter of the electron beam. In either method, since evaluation is performed in a state in which a beam diameter is applied to an edge and not deflected, an image can not be seen in this state and electron beam position fluctuations that are synchronous with the image are not obtained.
An apparatus for measuring a phase difference occurring between two electrical signals (reference signal and measurement signal) in a heterodyne interferometer is also known (for example, see Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 09-505143 A1 (WO 95/14236 A1)). That apparatus is a breakthrough invention that further divides the resolution of a heterodyne interferometer using a digital circuit. For example, with respect to a beat frequency of two megahertz, when the clock frequency is 40 megahertz that is twenty times the beat frequency in speed, the measurement resolution of the phase difference can be enhanced to 1/10, by 2× (beat frequency)/(clock frequency).